util: Add an API to atomic-replace a file, dirfd relative, optional fsync

This can be used in other places too, but I plan to use it to write
the summary file.

src/libotutil/ot-gio-utils.h

@@ -69,6 +69,13 @@ gboolean ot_gfile_load_contents_utf8_allow_noent (GFile          *path,
                                                   GCancellable   *cancellable,
                                                   GError        **error);
 
+gboolean ot_file_replace_contents_at (int             dfd,
+                                      const char     *path,
+                                      GBytes         *contents,
+                                      gboolean        datasync,
+                                      GCancellable   *cancellable,
+                                      GError        **error);
+
 gboolean ot_gfile_replace_contents_fsync (GFile          *path,
                                           GBytes         *contents,
                                           GCancellable   *cancellable,